[vbezhenar]

It's very simple API with very basic functionality and this guy probably already did some GUI applications, so he just put few images, buttons, forms and lists together.

Reverse engineering HTTP API is not hard either, open developer console, inspect API requests, responses, etc. You need to have some web development experience, I guess.

Just read any introductory book for OS X programming and you'll be able to build this app yourself.